Who is Participating?

Ok the first you need to do is to identify how you can filter this user from the rest, one option is to set custom attribute for instance Custom Attribute 3 as NotIncludedInDDG_Test1 (have a easy understandable name and this also gived you more options in the future)

Here you have all users which don't have CustomAttribute3 as NotIncludedInDDG_Test1.

Just short on what these arguments are.
- OrganizationalUnit: Specifies the container/OU where the group will be created
- RecipientContainer: Specifies container to pick up recipients from. If not specified, this gets set to the same value as the OrganizationalUnit parameter (the OU/Container where the group is created), and the filter may not return the expected recipients (or worse— may not return any recipients at all... )

Dynamic groups are created on-the-fly and the membership is evaluated according to the criteria set. If your dynamic group is set to contain all users, then by definition you can't remove a user from it.

Normally dynamic groups ae set to evaluate membership according to a specified field (such as all those iin a particular department). You will need to update the user accounts with appropriate information and define the query appropriately